Background

Orbiviruses are 1 of 9 genera in the family Reoviridae. Only 4 genera within the Reoviridae cause human disease: the orbiviruses, rotaviruses, orthoreoviruses, and coltiviruses. Coltiviruses cause Colorado tick fever. Orbiviruses are distinguished from the orthoreoviruses by their protein structure and arthropod transmission cycles.
[1] The genus Orbivirus contains 19 species and at least 130 subspecies.

Orbiviruses are named for their doughnut-shaped capsomeres (orbi means ring in Latin). Structurally, each virion consists of an outer and inner capsid layer that surrounds a core genome of 10 nonenveloped double-stranded RNA segments. Type- and group-specific antigens include the outer VP2 and the core VP7 proteins, respectively. No lipid envelope is present. Overall, they are 70-80 nm in diameter.

The orbiviruses are primarily animal pathogens that cause bluetongue disease in sheep, cattle, goats, and wild ungulates; African horse sickness in horses, donkeys, and dogs; and epizootic hemorrhagic deer fever. Many other viruses in this genus infect animals, but the above are the most commonly recognized. Infections may affect fetal development and have been linked to congenital anomalies such as hydrocephalus and arthrogryposis. These are the best-studied orbiviral diseases. Only 7 per 100 orbiviruses are linked to human clinical disease, with only one, Oklahoma tick fever, causing disease within the United States.

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Pathophysiology

Orbiviruses are vector-borne pathogens that are transmitted by ticks, mosquitoes, gnats, and midges. Studies on the pathophysiology of orbiviral infections specifically in humans have not been conducted. Data on the pathophysiology of orbiviral infections are derived mainly from studies of animal orbiviruses. According to Palacios et al, the ability to detect them has been hampered by their diversity. They developed a consensus reverse-transcription polymerase chain reaction (RT-PCR) method of targeting the polymerase gene for orbivirus recognition and characterization.
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In these cases, the virus enters the cell by endocytosis, the outer capsid is removed, and the core particle undergoes transcription. Viral proteins are synthesized 2-14 days after the infection and self-assemble within the cytoplasm. New virions are released and are then capable of infecting more cells, mainly within the lymphoreticular system. Many orbiviruses have a predilection for the vascular endothelial cells, which can lead to disruption in blood flow and subsequent ischemia. Orbiviruses also have a tropism for the nervous system, leading to encephalitis and, possibly, neuropathies.

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Epidemiology

Frequency

United States

Within humans, Oklahoma tick fever is a tick-borne febrile illness that may be caused by an Orbivirus in the Kemerovo group. The exact frequency is unknown, but clinically recognized disease is uncommon. Two serologically diagnosed human cases have been reported in Oklahoma and Texas. These cases involved elevated levels of antibodies to Six Gun City and Lipovnik viruses, which are members of the Kemerovo serogroup. Viral isolation was not performed; therefore, the precise identity of the cases is not definitively known.

International

Internationally, only infrequent cases of clinical illness in humans have been reported in Russia, Eastern Europe, Africa, and South and Central America. To date, fewer than 50 cases have been described in the literature.

Mortality/Morbidity

Deaths have not been attributed to orbiviral infections in humans. However, severe flulike illness, encephalitis, and polyradiculitis have occurred infrequently in patients with orbiviral infections. Patients with clinical orbiviral infections generally recover with no long-lasting effects.

Race

Current reports do not demonstrate a racial predilection for orbiviral infections; however, no large epidemiologic trials or case series have been performed. Further clarification of this point requires more clinical cases with in-depth epidemiological investigations.

Sex

In the few human cases of orbiviral infections, incidence does not seem to be associated with sex.

Age

All age groups may be infected with orbiviruses. However, seroprevalence studies suggest that most infections occur in childhood.

Chumakov MP. Report on the isolation from Ixodes persulcatus ticks and from patients in western Siberia of a virus differing from the agent of tick-borne encephalitis. Acta Virol. 1963 Jan. 7:82-3. [Medline].